According to the existing art, a thin film capacitor for use in an electronic circuit is made by sequentially depositing a lower conductor, a dielectric thin film, and an upper conductor on a silicon (Si) substrate. When a thin film capacitor is made by such a method, the thickness of the capacitor cannot be reduced to the thickness or less of the Si substrate, which does not contribute to capacitance. Thus, in order to promote further size and thickness reduction, a structure and a production method that do not require a substrate must be realized.
For example, patent document 1 proposes a thin film capacitor including a metal foil, an inorganic dielectric thin film deposited on the metal foil, and a metal body formed on the inorganic dielectric thin film.
According to patent document 1, the dielectric thin film and the metal body can be sequentially deposited on the metal foil by a vacuum process such as RF magnetron sputtering or vacuum deposition. Thus, there is no need to provide a substrate, such as a Si substrate, for deposition, thereby enabling size and thickness reduction of the thin film capacitor.
Patent document 2 proposes a method for making a ceramic multilayer substrate including the steps of press-bonding, on both sides of a green compact formed from a plurality of green sheets with conductor patterns, dummy green sheets having a sintering temperature higher than that of the green sheets, firing the green compact and the dummy green sheets, and removing the dummy green sheets on the two sides of the resulting sintered compact. Patent document 2, however, does not relate to the technical field of the thin film capacitors.
Patent document 2 describes an alumina green sheet as the dummy green sheet having a sintering temperature higher than that of the green sheet.
Patent document 1: Japanese Unexamined Patent Application Publication No. 8-78283
Patent document 2: Japanese Unexamined Patent Application Publication No. 9-249460
Patent document 1, however, described that, from the standpoint of simplifying the production process, it is preferable to use a vacuum process as the method for depositing the dielectric thin film or the metal body.
However, the vacuum process requires high process cost and high material cost for the metal foil. Thus, the production cost can increase sharply.
One conceivable method for reducing the production cost is to make a thin film capacitor by press-bonding conductor green sheets, which form conductor layers, on both main surfaces of a dielectric green sheet, which forms a dielectric layer to prepare a laminate, and firing the laminate.
However, since the capacitor portion formed from the dielectric green sheet and the conductor green sheets is usually a thin film of 100 μm or less in thickness, the capacitor portion would undergo warpage or undulation by thermal shrinking occurring during firing.
One possible method for preventing such warpage and undulation is a method described in patent document 2 of press-bonding alumina green sheets functioning as dummy green sheets on both sides of a laminate. In such a case, however, since alumina contained in the dummy green sheets adheres on the conductor green sheet during firing, it is necessary to remove alumina by conducting polishing or the like after the firing.
However, since the thickness of the thin film capacitor is 100 μm or less, the dielectric layer may be damaged and the electrical shorting may occur between conductor layers by polishing or the like. Moreover, damage on the dielectric layer caused by polishing severely increases as the thickness of the dielectric layer is reduced. This poses a serious impediment to reducing the size of and increasing the capacitance of the thin-film capacitor.
In order to prevent adhesion of alumina on the conductor green sheets, the particle size of alumina may be increased. However, this will cause a new problem, i.e., an increase in surface roughness of the main surfaces of the conductor green sheets.
Thus, it is difficult to obtain a thin film capacitor at low cost without imparting adverse effects to the dielectric layer by applying the method for making the ceramic multilayer substrate set forth in patent document 2 to the thin film capacitor.